A. Field of Invention
This invention relates to ceramic substrate wafers for electrical circuit boards and methods by which they may be secured to structural housings.
B. Description of Prior Art
Perhaps the commonest circuit board material to which electrical circuit components are presently mounted comprises a sheet of glass fiber-filled epoxy. Glass fiber epoxy is relatively nonbrittle and, thus, such a circuit board may be readily attached to a structural housing by simply drilling holes into the circuit board material and inserting metal screws, which may be tapped into the housing and tightened. The relative non-brittleness of glass fiber epoxy also permits use of such circuit boards in rather extreme mechanical and thermo-mechanical environments without fear of cracking the board.
However, there exist electronic circuit board applications in which use of a glass fiber epoxy board is undesirable. For example, if such a board is used in very high frequency (approximately L-band and higher) communication devices, the relatively low dielectric constant of the material causes the electronic circuit to become "lossy".
In such applications, a ceramic substrate wafer, typically fused alumina or sapphire, is ordinarily employed for circuit board construction. However, due to their brittleness, ceramic wafers have a strong tendency to split or shatter, when subjected to vibration and other ordinary mechanical stresses.
Mounting of ceramic substrate circuit boards to housing structures presents additional difficulties. Screw mounting of the board directly to the housing cannot be readily accomplished, since if the ceramic wafer is not split during the process of drilling the holes for passage of the screws, it will most likely be at least locally cracked when the screws are tightened. Adhesive bonding or soldering of the ceramic substrate wafer onto the housing, beside making removal difficult, presents problems in connection with thermal expansion of the housing at a different rate from that of the ceramic material typically resulting in mechanical distortion of the housing, causing buckling and cracking of the circuit board.